1. Field of the Invention
The present invention relates to apparatuses and methods for spot welding a plate workpiece in which a thicker plate and a thinner plate having different rigidities are stacked one on top of the other.
2. Description of the Related Art
In the art of joining stacked plates such as steel plates, spot welding has been widely used in which the plates are clamped between a pair of welding electrodes and a large current is passed between the electrodes for a predetermined time period while applying a welding pressure to the plates in order to heat the joint zone to substantially a melting temperature, thereby joining the plates together.
Provided that the welding pressure applied by the welding electrodes and the energization time in spot welding are constant, a nugget diameter gradually increases with an increase in current amperage. However, the application of an excessively large current increases the amount of generated heat, possibly resulting in a phenomenon called expulsion in which molten metal is expelled from between the plates. That is, expulsion is a phenomenon of molten metal exploding due to superheating at the joint zone and results in a nugget having defects such as holes and cracks. Such discontinuities in the nugget shape and the metal structure can be a factor that markedly decreases the joint strength as well as can be a factor reducing the plate thickness at the joint. On the other hand, the application of an excessively small current results in a small nugget that does not achieve sufficient joint strength. Further, when the welding pressure is low, the area of contact between the plates becomes small with the result that the current density is increased and expulsion is caused due to superheating. When the welding pressure is excessively high, the contact area at the joint becomes large and the current density is lowered with the result that the amount of generated heat is reduced. Consequently, the resultant nugget is small and does not achieve high weld strength.
Reference is now made to FIG. 13A which illustrates spot welding of a workpiece 100 in which a thinner plate 101 having a low rigidity and a small thickness, and a first thicker plate 102 and a second thicker plate 103 having a larger thickness, namely, a higher rigidity than the thinner plate 101, are stacked on top of one another. As illustrated, the workpiece 100 is clamped by a movable electrode 111 and a fixed electrode 112 in a condition such that the plates are in intimate contact without any clearance between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103. Energization by a power supply 113 results in a substantially uniform current density throughout an energization path between the movable electrode 111 and the fixed electrode 112. As a result, a satisfactory nugget is formed in a region ranging from the thinner plate 101 to the second thicker plate 103, thereby achieving desired weld strength.
In actuality, however, the low-rigidity thinner plate 101 and the first thicker plate 102 are bent upward when the workpiece 100 is clamped and pressed by the movable electrode 111 and the fixed electrode 112, leaving clearances between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103.
In such a case, the area of contact between the movable electrode 111 and the thinner plate 101 is increased due to the deflection of the thinner plate 101, while the contact areas at the joints between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103 are reduced by the clearances.
As a result, the current density between the movable electrode 111 and the fixed electrode 112 becomes higher on the second thicker plate 103 side than on the thinner plate 101 side, and the amount of local heat generated between the first thicker plate 102 and the second thicker plate 103 is larger than that generated between the thinner plate 101 and the first thicker plate 102.
Consequently, a nugget 105 is formed first at the joint between the first thicker plate 102 and the second thicker plate 103 as illustrated in FIG. 13A, and the nugget 105 gradually grows so as to weld the thinner plate 101 and the first thicker plate 102 as illustrated in FIG. 13B. However, the amount of penetration through the first thicker plate 102 into the thinner plate 101 is small and the weld strength is unstable. Thus, there is a risk that the thinner plate 101 will become separated. Further, the weld quality is variable. In particular, this defect is more likely with an increase in thickness of the first thicker plate 102 and the second thicker plate 103 because it is more difficult for the nugget 105 to reach between the first thicker plate 102 and the thinner plate 101.
Another reason for the small penetration between the thinner plate 101 and the first thicker plate 102 and the consequent unstable weld strength is the fact that due to the small thickness of the thinner plate 101, heat is drawn into the movable electrode 111 when the movable electrode 111 is in contact with the thinner plate 101 with the result that the temperature on the thinner plate 101 side is not increased, thereby preventing the formation of the nugget 105.
An exemplary remedy for this problem is a spot welding method disclosed in Japanese Unexamined Patent Application Publication No. 2003-251469. According to this spot welding method, as illustrated in FIG. 14, a workpiece 100 in which a thinner plate 101, a first thicker plate 102 and a second thicker plate 103 are stacked on top of one another is spot welded in a manner such that the welding pressure (FU) of a movable electrode 125 located on the thinner plate 101 side is controlled to be smaller than the welding pressure (FL) of a fixed electrode 124 positioned on the second thicker plate 103 side, whereby the contact resistance at the joint between the thinner plate 101 and the first thicker plate 102 is increased while the contact resistance at the joint between the first thicker plate 102 and the second thicker plate 103 is decreased. In this manner, energization between the movable electrode 125 and the fixed electrode 124 results in an increased amount of heat generated at the joint between the thinner plate 101 and the first thicker plate 102. Thus, the weld strength between the thinner plate 101 and the first thicker plate 102 can be increased.
FIG. 15 illustrates a structure of a spot welding gun that is used in performing the above method. As illustrated, a spot welding apparatus 120 is mounted on a wrist 116 of a welding robot 115. The welding robot 115 is configured to spot weld the workpiece 100 while moving the spot welding gun 120 to each of the welding sites on the workpiece 100 that is supported by a clamper 118.
The spot welding gun 120 has a base unit 122 that is vertically movably supported by a linear guide 121 fixed to a gun-supporting bracket 117 that is attached to the wrist 116. The base unit 122 is fitted with a C-shaped yoke 123 that extends downward. The fixed electrode 124 is disposed on the lower tip of the C-shaped yoke 123.
A pressure actuator 126 such as a servomotor is mounted on the upper end of the base unit 122. The movable electrode 125 opposing the fixed electrode 124 is attached to the lower end of a rod 127 that is vertically movable by the pressure actuator 126. A servomotor 128 is mounted on the upper end of the gun-supporting bracket 117. The base unit 122 is movable vertically by a ball screw mechanism by driving of the servomotor 128.
In accordance with teaching data stored in a controller which is not shown, the welding pressure (FU) of the movable electrode 125 located on the thinner plate 101 side is controlled to be smaller than the welding pressure (FL) of the fixed electrode 124 (EU<FL).
In order to control the welding pressure (FU) of the movable electrode 125 to be smaller than the welding pressure (FL) of the fixed electrode 124 (FU<FL), the controller first actuates the servomotor 128 so as to lift the base unit 122 and bring the fixed electrode 124 into contact with the lower surface of the workpiece 100. At the same time, the controller actuates the pressure actuator 126 so as to lower the movable electrode 125 into contact with the upper surface of the workpiece 100. In this case, the welding pressure applied by the pressure actuator 126 acts evenly on the movable electrode 125 and the fixed electrode 124 via the base unit 122 and the C-shaped yoke 123.
Next, the base unit 122 is lifted by the servomotor 128. By lifting the base unit 122, the welding pressure (FL) of the fixed electrode 124 is increased in accordance with lifting of the base unit 122. As a result, the welding pressure (FU) of the movable electrode 125 becomes smaller than the welding pressure (FL) of the fixed electrode 124 (FU<FL).
Consequently, when a current is passed between the movable electrode 125 and the fixed electrode 124, a high current density is achieved at the joint between the thinner plate 101 and the first thicker plate 102 and the amount of heat generated at the joint is increased relative to the amount of heat generated at the joint between the first thicker plate 102 and the second thicker plate 103. Thus, a uniform and satisfactory nugget is formed in a region ranging from the thinner plate 101 to the second thicker plate 103, and high weld strength is ensured.
According to Japanese Unexamined Patent Application Publication No. 2003-251469, the spot welding gun 120 is moved to each of the welding sites on the workpiece 100 that is supported by the clamper 118; the fixed electrode 124 is caused to abut against the second thicker plate 103 of the workpiece 100 and the movable electrode 125 is caused to abut against the thinner plate 101; and the base unit 122 is lifted to control the welding pressure (FU) of the movable electrode 125 to be smaller than the welding pressure (FL) of the fixed electrode 124. In this manner, the current density between the thinner plate 101 and the first thicker plate 102 is relatively increased to ensure that an appropriate amount of heat is generated at the joint between the thinner plate 101 and the first thicker plate 102. Thus, the amount of penetration is increased and the weld strength is improved.
However, the clamper 118 that clamps the workpiece 100 is required to withstand a heavy load when the welding pressure (FU) of the movable electrode 125 is controlled to be smaller than the welding pressure (FL) of the fixed electrode 124 by moving the base unit 122 while the workpiece 100 that is clamped by the clamper 118 is pinched between the fixed electrode 124 and the movable electrode 125. In the case where the position of the clamping of the workpiece 100 by the clamper 118 is very far from the welding position, namely, the position of a welding site, the workpiece 100 is distorted and deformed, whereby variations in the welding pressure (FL) of the fixed electrode 124 and the welding pressure (FU) of the movable electrode 125 are caused. As a result, it is difficult to stably ensure an appropriate contact resistance between the thinner plate 101 and the first thicker plate 102 and between the first thicker plate 102 and the second thicker plate 103. Thus, there is a risk that the current density at the joints in the workpiece 100 will be nonuniform and the quality of the spot weld will be lowered. Further, in the above method, a spot welding gun that has an equalizing function between the robot wrist and the base unit to allow for movements utilizing a reactive force produced by the welding pressure cannot be adopted. Thus, usable spot welding guns are limited.